Plain English Meaning
When the airplane starts going somewhere the pilot did not tell it to go — a wandering heading, a creeping altitude, a slow roll, a pitch change — and it happens on its own.
Full Definition
Unintended departures of an aircraft from its intended flightpath, attitude, airspeed, or altitude. In Upset Prevention and Recovery Training (UPRT), the term refers specifically to deviations that occur without the pilot commanding them, including drift toward an unusual attitude, an unintended pitch or bank change, or an airspeed or altitude trend that was not selected.